A membrane is a discrete, thin interface that moderates the permeation of chemical species in contact with it. Water filtration membranes allow water to penetrate through the membrane while preventing penetration of target species. Solutes such as colloids, bacteria, viruses, oils, proteins, salts, or other species can be removed using a membrane. Polymer filtration membranes can be categorized into porous and nonporous membranes, referring to the porosity of the selective layer of membranes prepared by the immersion precipitation process. In porous membranes, the transport barrier posed is based on differences between the sizes of permeate and retentate species. In nonporous membranes, such as those used for reverse osmosis, the species are separated by means of relative solubility and/or diffusivity in the membrane material. For nonporous membranes and porous membranes for nanofiltration, poor chemical affinity between the membrane material and permeate that is passed across the membrane material, e.g., water, may inhibit permeability of the permeate. Important parameters that can characterize a good membrane for liquid filtration include high flux, fouling resistance, and/or selectivity in the desired size range. An improvement in these properties can lead to improved membrane performance.
A membrane exhibiting high flux may decrease the cost of energy for pumping the solution through the membrane, which can make the process economical. Membranes that exhibit more uniform pore sizes can have higher selectivity and/or higher efficiency.
Membrane fouling is one of the more important problems in the membrane industry. It can generally be characterized by a decline in membrane flux over time caused by components in the feed solution passed across the membrane. It can occur due to the adsorption of molecules on pore walls, pore blockage, or cake formation on the membrane surface. Flux decline typically leads to higher energy requirements, and frequent cleaning is usually required to remedy this. This is only a temporary solution, and fouling typically ultimately reduces the lifetime of the membrane. As fouling often involves the adsorption of biomolecules to the membrane surface, it can also reduce the biocompatibility of the membranes in biomedical applications.
It has been observed that hydrophilic membrane surfaces foul less, especially in membranes with larger pore sizes such as those used in ultrafiltration (UF) and microfiltration (MF). Greater wettability may reduce adsorption on the membrane surface of species present in the solution. One method of making surfaces hydrophilic is using oxygen plasma treatment, which generates hydrophilic groups such as hydroxyl and carboxylic acid groups on the material's surface. This method, however, only functionalizes the top surface of the membrane, so the fouling of internal pores is not prevented.
Graft polymerization of hydrophilic groups onto membrane surfaces has been the most common method of increasing the fouling resistance of membranes. A variety of hydrophilic monomers have been grafted onto different synthetic membranes to limit fouling by natural organic matter (NOM) and proteins. A significant drawback of these surface modification methods is the use of high-energy gamma radiation or plasmas to initiate graft polymerization. These approaches may significantly increase membrane fabrication cost and are poorly controlled. Undesirable side reactions include polymerization of ungrafted chains, which are susceptible to removal from the surface during use. Such surface graft-polymerized layers can also block pores and deteriorate flux.
Another method involves incorporation of an additive that contains reactive groups during the casting of the membrane, followed by the chemisorption of a hydrophilic and preferably biocompatible polymer through these reactive groups. A similar method involves the activation of a fraction of the groups on the surface of a cast polymer membrane (e.g. the nitrile groups in polyacrylonitrile) by a reagent, followed by coupling of a hydrophilic polymer, resulting in surface grafts. After casting the membrane, some of the nitrile groups may be converted to intermediate reactive sites followed by the coupling of poly(ethylene glycol) chains to form grafts on the surface. These methods all include several extra processing steps for the activation of reactive sites and coupling, and therefore can be relatively expensive.
Another method of producing hydrophilic surfaces is the surface segregation of polymeric additives during membrane casting by immersion precipitation. Such a method does not require any significant additional processing steps in membrane fabrication, limiting cost and providing easy integration with existing membrane casting processes. Such methods have been proposed using relatively hydrophilic homopolymers or using block copolymers with a hydrophilic and a hydrophobic block.
Others have previously used two graft copolymer additives comprising poly(oxyethylene) methacrylate, POEM, namely poly(vinylidene fluoride)-g-POEM and poly(methyl methacrylate-ran-POEM), previously to produce ultrafiltration membranes that resist protein fouling. Nevertheless, the ultrafiltration membranes produced still did not resist fouling completely and some irreversible flux loss was observed in studies using protein-containing feed solutions.